Glycidyl esters of aromatic carboxylic acids can be employed in many ways in the adhesive, synthetic resin and lacquer industries and have been claimed as cross-linking components, e.g., in diverse patents for the production of heat-hardenable coating compositions.
Additionally, there are known several methods for the production of these compositions. As is mentioned in Hao, Die Angewandte Makomolekularen Chemie Vol. 31, pages 83-113 (1973), glycidyl esters can be produced by:
1. Reaction of epichlorohydrin with carboxylic acids and subsequent dehydrohalogenation Japanese application No. SHO 55-127380 U.S. Pat. No. 3,576,827, German AS No. 2,126,280, German AS No. 1,643,777.
2. Reaction of epichlorohydrin with salts of carboxylic acids, Hao, Die Angewandte Makromolekularen Chemie Vol. 31 (1973), pages 83-113 (No. 443).
3. By epoxidation of allyl esters, EPO published application No. 0008,112.
4. By reaction of acid chlorides with 2,3epoxypropanol (glycidol), Sandler, J. Chem. Eng. Data Vol. II, No. 3, pages 447-448.
Furthermore, there may be mentioned the following possibilities described in the literature.
5. Reaction of an acid hydride with epichlorohydrin.
6. Transesterification of acid esters with glycidyl esters, German OS No. 2107084.
7. Transesterification of acid esters with glycidol, Russian patent No. 405,880, German OS No. 2,602,157.
All described methods up to now are neither suited for large scale manufacture or are associated with various disadvantages which affect the quality of the end product.
The dehydrohalogenation of the 2,3-hydroxychloropropyl esters, which are obtained by reaction of the carboxylic acid with epichlorohydrin, with an alkaline reagent (e.g., German AS No. 1,030,824 and Dukes U.S. Pat. No. 3,576,827) or by re-epoxidation with further epichlorohydrin (German AS No. 1,643,777 and German AS No. 2,126,280) in the first case with mild reaction conditions does not lead to complete splitting off of Cl, but with stronger conditions leads to a breaking of the ester bond so that the glycidyl ester formed again splits off (see German AS No. 1,030,824). In the second case, as generally is the case in the reaction with epichlorohydrin, it is not possible to arrive at chloride free end products, the process produces as is the case with the reaction of carboxylic acid anhydride with epichlorohydride (Japanese application SHO 55-127380 large amounts of organic loaded salt.
Besides, all of the mentioned processes are expensive in their industrial application because the reaction of the alkali salt with epichlorohydrin (e.g., German patent No. 1,081,013 and Kester U.S. Pat. No. 2,448,602) brings about no substantial advantages, especially since a further process step is needed, namely the production of the carboxylic acid salt involved. The content of residual chlorine in all the processes is quite high and can amount to several percent. The epoxidation of the allyl ester with per compounds is mentioned in several patents (see, e.g., Great Britain patent No. 862,588, German AS No. 1,082,263, S. R. Sandler and F. R. Berg, J. Chem. and Eng. Data, Vol. 11, pages 447-448 (1966) and European published application No. 0008112).
Until now, these methods have only been used to a limited extent since the processes are expensive and technologically difficult. The reaction of acid chlorides with glycidol because of the poor availability of the chloride and the high accumulation of alkali or amine salts brings no advantages in regard to quality and simplicity of the course of the reaction (see, e.g., F. Zetche and F. Aeschlimann, Helv. Chim. Acta Vol. 9, pages 708-714 (1929), Raecke U.S. Pat. No. 3,073,804).
The transesterification of acid esters with lower molecular weight aliphatic glycidyl esters (German OS No. 2,654,306 and German OS No. 2,107,084) is indeed possible, but the process fails for large scale manufacture because of the poor availability of the aliphatic glycidyl esters.
Glycidyl esters can also be produced by reaction of glycidol with esters of aromatic carboxylic acid, preferably the methyl esters.
This process is technically elegant and of little expense in regard to the apparatus and working up of the mother liquor. Besides, the necessary starting components are available on an industrial scale.
Until now, there has only unsatisfactorily been solved the question of usable catalysts. In the USSR patent No. 405,880, there is claimed zinc acetate. However, the stated reaction temperatures of 100.degree.-150.degree. C. led to a quick polyaddition of the glycidol. In reworking the stated process, there were always obtained only small amounts of diglycidyl ester, besides much starting product, there was ascertained a considerable polymer portion.
Zondler et al describe in Helv. Chim. Acta. Vol. 60, pages 1845-1860 (1977), as well as in the German OS No. 2,602,157, the transesterification of carboxylic acid methyl esters with glycidol in the presence of different thallium salts as catalysts.
Their investigations of the systems HgO, CdO, PbO, PbO.sub.2, Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, Ga.sub.2 O.sub.3, In.sub.2 O.sub.3, Mn(ac).sub.2, Hg(ac).sub.2, Pb(ac).sub.2, UO.sub.2 (ac).sub.2, TiO(acac).sub.2, In(acac).sub.3, Th(acac).sub.4, Ga(O-n-C.sub.4 H.sub.9).sub.4, Ti(O-n-C.sub.4 H.sub.9).sub.4, Ti(O-iso-C.sub.3 H.sub.7).sub.4, (n-C.sub.4 H.sub.9)Sn(ac).sub.3, (n-C.sub.4 H.sub.9).sub.3 Si-ac, n-C.sub.4 H.sub.9 -Sn-OOH and KCN show that all catalysts show no or little activity below 100.degree. C. Therefore, there results only low degree of reaction.
The thallium containing catalysts such as TlNO.sub.3, Tl.sub.2 O.sub.3, TlOCOCH.sub.3 among others claimed in the above German Offenlengungsschrift indeed show a good activity, but it is practically not possible to isolate the glycidyl ester formed in thallium free form. Because of the known toxicity of this metal and its derivatives, the employment of such thallium containing esters is greatly limited.
The invention, therefore, is based on the problem of finding catalysts for the technically simple process of transesterification of methyl esters with glycidol which permits the production of glycidyl esters on a large scale because of their activity and low toxicity.
Precisely, the transesterification of polybasic carboxylic acid esters is a problem since it concerns further reacting the product of the intermediately occurring intermediate step, i.e., the mixed ester in the direction of the pure glycidyl ester. The catalysts used for the reaction of monoesters are not suited without doing anything else for reacting polyester. Rather, it has been shown that the catalysts claimed for aromatic carboxylic acid monoesters such as NaOCH.sub.3 (German OS No.2,107,084), other basic materials such as hydroxides or cyanides, ion exchangers, acids such as H.sub.2 SO.sub.4, p-toluenesulfonic acid are not suited to produce pure polyglycidyl esters.
The reaction with glycidol many times only leads to the formation of the monoglycidyl ester, e.g., there is obtained in the transesterification of dimethyl terephthalate with 2 moles of glycidol in the presence of KCN chiefly the monoester, besides the starting product and much polymeric material.
A large number of alkali salts which were examined in regard to the catalytic activity proved to be not suited. There belongs to this group among others the anions of the following acids:
PO.sub.4.sup.3-, HCO.sub.3.sup.-, HPO.sub.4.sup.2-, NO.sub.3.sup.-, NO.sub.2.sup.-, BO.sub.2.sup.-, H.sub.2 PO.sub.2.sup.-, BH.sub.3 CN.sup.-, SO.sub.4.sup.2-, SO.sub.3.sup.2-, IO.sub.4.sup.-, S.sub.2 O.sub.8.sup.2-, S.sub.2 O.sub.5.sup.2-, S.sub.2 O.sub.4.sup.2-.
The problem of the process of the invention, therefore, is to produce aromatic mono, and above all polycarboxylic acid glycidyl esters by reaction of aromatic carboxylic acid alkyl esters with glycidol in very good yields and high purity, namely using environmentally favorable catalysts.